Conventional water heaters provide heated water by storing heat energy in the water. Because the heat energy is stored in the water, a large portion of the heater must be dedicated to storing the heated water until it is used. Typically, water stores approximately 10 calories/gram of heat per 10xc2x0 C. increments. By requiring a large portion of the heater for storage of heated water, the water heater must be of reasonable size to store enough heated water for use in a standard residential environment. Typical residential water heaters have a 50-60 gallon capacity. For use in buildings and industry, water heaters must be even larger.
Water heaters consume a high percentage of residential energy heating water for bathing, washing dishes, washing clothes and heating homes and offices. In homes heated by electricity, the consumption of electric power is even greater. Overall, a large imbalance in electric power usage exists during the day time due primarily to the large amounts of power consumed by industry, businesses and public transportation. To compensate for the extensive day time use of electric power, utility companies provide generating capacity sufficient to supply day time usage, leaving unused capacity available for the night hours.
Thus, a need has arisen in the art for a water heater which can more efficiently heat water, which can make effective use of utilities during off-peak hours to minimize building and household power consumption and which would consequently reduce building and household utility costs.
Water heaters are known which utilize phase change materials to heat water more effectively. Such phase change materials have a latent heat which is greater than the sensible heat of liquid water. A water heater utilizing a phase change material is described in my U.S. Pat. No. 6,047,106. The heater includes heat exchange tubes positioned in a heat exchange unit containing the phase change material such that the heat stored in the phase change material can be transferred to water.
However, a disadvantage of the use of phase change materials is that they expand upon melting and contract upon freezing. Where neat phase change materials are used, a substantial change in volume occurs which can lead to rupture of the container in which the phase change materials are held. It is possible to eliminate significant changes in volume during thermocycling by using modified formulations of phase change materials such as a mixture of a phase change material/silica dry powder, or a melt mix of phase change material/high density polyethylene/ethylene vinyl acetate/silica. However, such mixes contain only about 60% by weight phase change material. Compositions containing a higher content of phase change materials are desirable in order to provide a higher level of thermal storage.
Accordingly, there is still a need in the art for a water heater utilizing a phase change material to effectively heat water which provides improved thermal storage without the problems of volume changes due to melting and freezing.
The present invention meets that need by providing a water heater which can more effectively heat water by utilizing greater amounts of phase change materials in a container in which heating of the phase change material is initiated at the top of the unit and cooling is initiated at the bottom of the unit. By operating in this manner, the volume of the phase change material in the container is effectively controlled so to as to provide sufficient space for expansion and contraction.
According to one aspect of the present invention, a water heater for heating water is provided comprising a source of water, a top water reservoir and a bottom water reservoir, a heating element for heating the water, and a heat exchange unit having an upper section and a lower section and outer walls, where the heat exchange unit contains a phase change material therein. A plurality of heat exchange tubes are positioned in the heat exchange unit with the phase change material located between and around the heat exchange tubes to substantially fill any spaces between the heat exchange tubes. The tubes are in heat transfer relation to the phase change material and in fluid connection with the source of water so that the water heated by the heating element flows through the tubes and heats the phase change material. The heat stored in the phase change material is then transferred through the tubes to water flowing therethrough at nearly constant temperatures equivalent to the freezing/melting temperature of the phase change material.
The water heater preferably further includes a shell having an exterior surface, where the heat exchange unit is enclosed in the shell, and a layer of insulation is included on the exterior surface of the shell. Preferably, the insulation has an xe2x80x9cRxe2x80x9d value of at least about 10. The insulation is preferably vacuum panel insulation having an xe2x80x9cRxe2x80x9d value of at least about 20 per inch of thickness.
The heating of the phase change material is initiated in the upper section of the heat exchange unit. In one embodiment of the invention, the phase change material is heated by heating the water in the top water reservoir. In this embodiment, the heating element comprises a resistance heating element positioned in the top water reservoir.
In an alternative embodiment of the invention, the heating element comprises a plurality of resistance heating elements positioned on the outer walls of the heat exchange unit. The heating elements are activated sequentially from the top to the bottom.
By heating the phase change material from the top to the bottom, the phase change material melts and expands into free space at the top of the unit.
The phase change material preferably comprises 100% by weight of a linear crystalline alkyl hydrocarbon having a melting/freezing temperature from about 20xc2x0 C. to 100xc2x0 C., and more preferably, from about 30xc2x0 C. to 90xc2x0 C.
In a preferred embodiment, the phase change material includes from about 2 to 10% by weight of a polymer selected from the group consisting of EPDM, polyisobutylene, polyisoprene, polybutadiene, chlorinated polyethylene, polyalkyl acrylate, and polyalkyl methacrylate rubber. More preferably, the phase change material includes from about 2 to 10% by weight of a polymer selected from the group consisting of low density polyethylene, crystalline ethylene/propylene copolymers, ethylene/vinyl acetate copolymers, ethylene/methyl acrylate copolymers ethyl/methacrylate copolymers, ethylene/glycol polymers, and polyethylene oxide. The polymers act as a thickening agent to reduce the potential for leakage.
In an alternative embodiment of the invention, the phase change material is in the form of a gel comprising from about 80 to 90% by weight of a linear crystalline alkyl hydrocarbon and about 10 to 20% by weight silica particles.
In another alternative embodiment, the phase change material is imbibed in a porous material such as an open cell foam.
The phase change material may also include from about 5 to 15% by weight carbon black to increase the thermal conductivity of the phase change material.
The phase change material may also include from about 5 to about 50% by weight of a metal selected from the group consisting of aluminum and copper in order to improve heat transfer.
Accordingly, it is an object of the present invention to provide a water heater which employs large amounts of a phase change material which is heated from top to bottom. These, and other objects and advantages of the present invention, will become apparent from the following drawings, detailed description and claims.